Polymers may be produced by solution polymerization, wherein monomer is polymerized in an inert solvent or diluent. The solvent may serve to solubilize the reactants and products, act as a carrier for the reactants and products, aid in the transfer of the heat of polymerization, and help in moderating the polymerization rate. The solvent may allow easier stirring and transferring of the polymerization mixture (also called cement). Nevertheless, the presence of solvent can present a number of difficulties. The solvent must be separated from the rubber and then recycled for reuse or otherwise disposed of. The cost of recovering and recycling the solvent adds to the cost of the rubber being produced, and there is the risk that the recycled solvent after purification may still retain some impurities that will poison the polymerization catalyst. In addition, some solvents, such as aromatic hydrocarbons, can raise environmental concerns. Further, the purity of the polymer product may be affected if there are difficulties in removing the solvent.
Polymers may also be produced by bulk polymerization (also called mass polymerization), wherein the monomer is polymerized in the absence or substantial absence of any solvent, and, in effect, the monomer itself acts as a diluent. Since bulk polymerization is essentially solventless, there is less contamination risk, and the product separation is simplified. Bulk polymerization offers a number of economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate. The solventless feature also provides environmental advantages, with emissions and waste water pollution being reduced.
Despite its many advantages, bulk polymerization requires careful temperature control, and there is also the need for strong and elaborate stirring equipment since the viscosity of the polymerization system can become very high. In the absence of added diluent, the cement viscosity and exotherm effects can make temperature control difficult. Consequently, local hot spots may occur, resulting in degradation, gelation, and/or discoloration of the polymer product. In the extreme case, uncontrolled acceleration of the polymerization rate can lead to disastrous “runaway” reactions. Because of these difficulties, bulk polymerization has not been widely utilized in the commercial production of synthetic rubbers.
Due to the need to control polymer cement viscosity and exotherm, bulk polymerization may be run to substantially less than 100% monomer conversion, and the remaining monomer is recycled. As in the case of solution polymerization, it is generally necessary to add a quenching agent to the polymerization mixture to inactivate the reactive polymer chains, the catalyst and/or the catalyst components prior to the isolation and drying of the polymer product. Without adding the quenching agent, unwanted side reactions such as crosslinking, coupling, and oxidation reactions can occur when the living or pseudo-living polymer and the active catalyst or catalyst components are exposed to air during the isolation and drying of the polymer.
Various quenching agents have been used in the prior art. Water, alcohols, and carboxylic acids have been commonly used, but they can contaminate the recycled monomer stream due to their low boiling points and high vapor pressures. To alleviate the monomer contamination problem, long-chain alcohols such as n-octanol and long-chain carboxylic acids such as 2-ethylhexanoic acid (EHA) and distilled tall oil (DTO), which have higher boiling points and lower vapor pressures, may be used. Unfortunately, due to the high molecular weights of these long-chain alcohols and carboxylic acids, high loadings of quenching agents are often required to effectively quench the polymerization mixture. These high loadings can result in the formation of high levels of metal alkoxide or metal carboxylate salts as foreign substances in the polymer product. The presence of these foreign substances can adversely affect the rubber vulcanization rate as well as the properties of rubber vulcanizates. For example, the presence of high levels of the metal salts can give poor vulcanizate properties.